Thoughts

For new services, make multiple accounts the rule, not the exception. Example: google calendar shows 1 user at a time by default, where you could theoretically be logged in as your work user, your personal user, and something else all simultaneously. Grind https://calendar.google.com/calendar/u/2/r into https://app.mycalendar.com

Other principles: I was thinking about this last night. I'm sure something will annoy me and then I'll update this

Be able to build up from scratch; make clear the primitives and how an application builds on top of it for good documentation

for a new command line: compatibility, debuggability, performance

click a link to open

don't burden the user with unnecessary distractions

batteries included